Add support for adding Library Benefits to a screener

As a Benefits Analyst
When I am on the Manage Benefits screen
Then I should be able to import from a library of pre-existing benefits to add to this screener
And still be able to create a new Custom Benefit from scratch if I so choose.

- imported benefits become a benefit like any other for the screener; they consist of library eligibility checks and the benefit's checks may be edited after import (including anything you could do for a "custom" benefit - add checks, remove checks, rename checks, associate custom checks)

In other words, an analyst can add Library Benefits or Custom Benefits to a screener. Both types of benefits behave the same after being added to a given screener.

Start at the Manage Benefits screen and trace the existing flow for creating a Custom Benefit. Review how library eligibility checks and benefit checks are represented, then verify that imported benefits can be edited like custom benefits, including adding, removing, renaming, and associating custom checks.

Written by the indexing model from the issue text.